Late one night, Sarah and Maria gaze at the stars above Number 13, Bannerman Road and contemplate the amazing adventures that they have shared. Elsewhere, at the Tycho Project Radio Telescope located near the village of Goblin’s Copse, Professor Nicholas Skinner and his daughter Lucy are astonished to detect cyclic wave patterns emanating from the vicinity of the planet Rigel Beta V – a signal that indicates intelligent life. Rushing outside, the Professor and his daughter then see two football-sized balls of blazing light whizzing around the telescope’s dish; when the strange lights hurtle off into the nearby forest, Skinner sets off in pursuit – but as he disappears into the pitch black beyond the trees, something terrifying emerges from the darkness and attacks Lucy… The following morning, Maria and her father receive a letter bringing news that will change their lives. Meanwhile, Sarah is annoyed to find her adopted son Luke and his friend Clyde apparently using her alien supercomputer, Mr Smith, to play war games, but she is swiftly placated when the boys explain that they are re-enacting the Battle of Waterloo for their history project. Having heard a news report of strange lights near Goblin’s Copse, Sarah asks Mr Smith if he has detected any alien activity in the area, but the computer has no information to support her suspicions. Sarah decides to investigate anyway, and as she makes for her car with Luke and Clyde in tow, she is joined by Maria. The quartet of adventurers soon arrive at the radio telescope, which Sarah explains has been set up at a converted Cold War listening post. Entering the building, Sarah and her friends are intrigued to find the place deserted – they are unaware that something is watching them through alien eyes from within the depths of the forest… Just as Luke accesses the project’s computer and discovers the message received from Rigel Beta V, a dishevelled Lucy bursts in, babbling that something in the woods has taken her father; whatever it was chased her too, until she fell and banged her head… Clyde offers to search the woods, but Sarah instructs him and Luke to search the complex’s buildings instead. Maria finally tells Sarah that her father has been offered a job at his company’s head office in Washington; Maria is sad at the prospect of leaving her friends, but is puzzled when Sarah just encourages her to go. After Clyde leads Luke into the forest, the two boys soon realise that they are being followed by something cloaked in an electrostatic field, a device that renders its wearer invisible. The creature suddenly appears before the boys and drops its cloak – revealing itself to be a heavily armed Sontaran warrior, who immediately opens fire on them! Fleeing for their lives, the resourceful Clyde and Luke manage to evade their attacker; however, they are now lost in the depths of the forest; looking for a way back to the base, the boys run into another, larger, cloaked object sitting amongst the trees… Back at the control room, Lucy tells Sarah and Maria about the lights she saw circling the dish, which Sarah reasons to be mechanical drones. They are interrupted by the arrival of Professor Skinner, who now seems distant and emotionless, dismissing the phenomenon as merely ball lightning. After telling Sarah and Maria to leave, the professor returns to his work at his computer; but then, after his visitors have left, the professor turns on his own daughter... After telling Maria that something is definitely wrong with the professor, Sarah receives a phone call from Luke; driving into the woods, she and Maria home in on Luke and Clyde, who lead them to the invisible object they have found; Sarah disrupts its perception camouflage matrix with her sonic lipstick – and is shocked when the spherical shape of a Sontaran ship shimmers into view. Realising the enormity of the threat they face, Sarah urgently orders her friends back to the car so that they can contact U.N.I.T. for help; unfortunately they are stopped by the arrival of the Sontaran, who levels his gun and declares them his prisoners. Proudly declaring himself to be Commander Kaagh‘The Slayer’, of the Tenth Sontaran Battlefleet, the Sontaran explains that he used his drones to lure the professor and his daughter out of the base. Maria manages to create a diversion, allowing him and her friends to make a run for it while the Sontaran is distracted; Luke and Maria flee into the forest, while Sarah and Clyde head for the Tycho Project to rescue the Skinners. Arriving in the control room, Sarah discovers that that the professor is under Kaagh’s control via a neural implant on his neck, but before she can remove the device she and Clyde are once again captured by Kaagh. Finding a ventilation outlet amongst the trees, Maria and Luke see a way to infiltrate the base undetected. Kaagh is intrigued to learn that Sarah has met his race before, and that knows of their war against the Rutan horde. When Kaagh reveals that he is the sole survivor of an invasion force that attempted to take over the Earth using sabotaged ATMOS devices, Sarah is delighted to learn that it was the Doctor who thwarted the aliens’ plans. Vowing to take Sarah back to Sontar to pay for her friend’s crimes, Kaagh explains his survival: despatched to Earth just as the Sontaran mothership was destroyed, his ship was caught in the explosion and sent spinning out of control towards the planet below.Although, Kaagh managed to recover from his injuries and repair his craft, his face still carries the scars of his defeat, and he has now formulated a plan for revenge: under Kaagh’s mind-control, Professor Skinner will hack into the systems of the three thousand satellites in orbit around the Earth, and in just forty-five minutes, when the satellites are in their optimum position, the commander will use the radio telescope to trigger a signal that will send them crashing down to Earth, smashing them into the planet’s nuclear reactors and rendering the Earth a smoking cinder. To while away the remaining minutes, Kaagh decides to experiment on Sarah and Clyde – the Sontaran raises his gun and shoots Sarah point blank…